数据库中的一对多，多对多实例

一对多关系 

　　例如:一个人可以拥有多辆汽车，要求查询某个人拥有的所有车辆。

//建立人员表
CREATE TABLE people(
    id VARCHAR(12) PRIMARY KEY,
    sname VARCHAR(12),
    age INT,
    sex CHAR(1)
);
INSERT INTO people VALUES('H001','小王',27,'1');
INSERT INTO people VALUES('H002','小明',24,'1');
INSERT INTO people VALUES('H003','张慧',28,'0');
INSERT INTO people VALUES('H004','李小燕',35,'0');
INSERT INTO people VALUES('H005','王大拿',29,'1');
INSERT INTO people VALUES('H006','周强',36,'1');
 //建立车辆信息表
CREATE TABLE car(
    id VARCHAR(12) PRIMARY KEY,
    mark VARCHAR(24),
    price NUMERIC(6,2),
    hid VARCHAR(12),
    CONSTRAINT fk_people FOREIGN KEY(hid) REFERENCES human(id)
);
INSERT INTO car VALUES('C001','BMW',65.99,'H001');
INSERT INTO car VALUES('C002','BenZ',75.99,'H002');
INSERT INTO car VALUES('C003','Skoda',23.99,'H001');
INSERT INTO car VALUES('C004','Peugeot',20.99,'H003');
INSERT INTO car VALUES('C005','Porsche',295.99,'H004');
INSERT INTO car VALUES('C006','Honda',24.99,'H005');
INSERT INTO car VALUES('C007','Toyota',27.99,'H006');
INSERT INTO car VALUES('C008','Kia',18.99,'H002');
INSERT INTO car VALUES('C009','Bentley',309.99,'H005');

多对多关系　

　　例如：学生选课，一个学生可以选修多门课程，每门课程可供多个学生选择

//建立学生表
CREATE TABLE student(
    id VARCHAR(10) PRIMARY KEY,
    sname VARCHAR(12),
    age INT,
    sex CHAR(1),
    class VARCHAR(6)
);
INSERT INTO student VALUES('S0001','王军',20,1,'c101');
INSERT INTO student VALUES('S0002','张宇',21,1,'c101');
INSERT INTO student VALUES('S0003','刘飞',22,1,'c102');
INSERT INTO student VALUES('S0004','赵燕',18,0,'c103');
INSERT INTO student VALUES('S0005','曾婷',19,0,'c103');
INSERT INTO student VALUES('S0006','周慧',21,0,'c104');
INSERT INTO student VALUES('S0007','小红',23,0,'c104');
INSERT INTO student VALUES('S0008','杨晓',18,0,'c104');
INSERT INTO student VALUES('S0009','李杰',20,1,'c105');
INSERT INTO student VALUES('S0010','张良',22,1,'c105');

    //建立课程表
CREATE TABLE course(
    id VARCHAR(10) PRIMARY KEY,
    sname VARCHAR(12),
    credit NUMERIC(2,1),
    teacher VARCHAR(12)
);
INSERT INTO course VALUES('C001','Java',3.5,'李老师');
INSERT INTO course VALUES('C002','高等数学',5.0,'赵老师');
INSERT INTO course VALUES('C003','JavaScript',3.5,'王老师');
INSERT INTO course VALUES('C004','离散数学',3.5,'卜老师');
INSERT INTO course VALUES('C005','数据库',3.5,'廖老师');
INSERT INTO course VALUES('C006','操作系统',3.5,'张老师');

    //建立选修表
CREATE TABLE sc(
    sid VARCHAR(10),
    cid VARCHAR(10)
);

ALTER TABLE sc ADD CONSTRAINT pk_sc PRIMARY KEY(sid,cid);
ALTER TABLE sc ADD CONSTRAINT fk_student FOREIGN KEY(sid) REFERENCES student(id);
ALTER TABLE sc ADD CONSTRAINT fk_course FOREIGN KEY(cid) REFERENCES course(id);

INSERT INTO sc VALUES('S0001','C001');
INSERT INTO sc VALUES('S0001','C002');
INSERT INTO sc VALUES('S0001','C003');
INSERT INTO sc VALUES('S0002','C001');
INSERT INTO sc VALUES('S0002','C004');
INSERT INTO sc VALUES('S0003','C002');
INSERT INTO sc VALUES('S0003','C005');
INSERT INTO sc VALUES('S0004','C003');
INSERT INTO sc VALUES('S0005','C001');
INSERT INTO sc VALUES('S0006','C004');
INSERT INTO sc VALUES('S0007','C002');
INSERT INTO sc VALUES('S0008','C003');
INSERT INTO sc VALUES('S0009','C001');
INSERT INTO sc VALUES('S0009','C005');